The present invention concerns a method in a drying group or groups provided with a single-wire draw in a multi-cylinder dryer of a paper machine for supporting a web, the multi-cylinder dryer comprising drying cylinders heated by means of steam or equivalent, and the web being pressed by the drying wire directly against the cylinder faces of the drying cylinders. Furthermore, the multi-cylinder dryer comprises leading cylinders or rolls, on which the web remains outside of the drying wire.
Further, the present invention concerns a device which is applied in the area of a single-wire draw in a multi-cylinder dryer of a paper machine, this dryer comprising a line of heated drying cylinders, preferably upper cylinders, and a line of leading cylinders or rolls, the web running between these lines while being supported by a drying wire, e.g. a felt, so that the web is pressed by the drying wire on the heated drying cylinders into direct contact with the heated face of these drying cylinders, while the web is situated on an outer face of the drying wire over the leading cylinders or rolls.
The present invention also concerns a cylinder used in the draw of a paper web.
A problem in the case of leading rolls of single-wire groups in a drying section of a paper machine, has been making a paper web adhere to the wire on the lower face of the cylinder. On the lower cylinders or leading rolls, the web runs as a topmost layer, while the wire remains between the web and an outer face of the leading cylinder or roll. This problem does not occur in the case of the other rolls in a single-wire group, since the paper web runs between the wire and the mantle face of the cylinder.
Attempts have been made to solve this problem by utilizing suction rolls. A suction roll is quite expensive, with a suction box requiring inner sealing ribs which tend to become worn. When the sealing ribs operate dry, the wearing is even more extensive.
Negative pressure is applied through the suction box of a suction roll to a vacuum block inside the cylinder. This vacuum block is placed in a lower half of the cylinder in the case of the lower cylinders in a single-wire group. Through the negative pressure applied to this block, suction is expressly applied to a part of the cylinder face in which the paper web tends to become detached. Through bores in the mantle, negative pressure is applied through the felt to the paper web, which thereby adheres to the felt face.
A solution in the prior art for this problem of the paper web tending to become detached, is also known in which an arrangement in accordance with Valmet FI Pat. Appl. 851533 is used. In the noted Uno-Vac system, a type of suction box is used in which no wearing seals placed against the inner face of the roll mantle are used. In this solution, a high-velocity air flow is applied to the proximity of the edge of the suction box and the mobile mantle face. This air flow generates a suction flow from the interior of the box, which thereby prevents flow of air through the edge area in the opposite direction into the space of negative pressure. A negative pressure is applied to the interior of the suction box and is transferred through the bores in the mantle to the web face. The high cost of construction of the equipment is considered a drawback of this solution. An abundance of holes must be made into the mantle. The inner axle requires flow ducts of its own, and suction ducts of its own.
The use of a suction box outside the mantle is also known in conjunction with single-wire groups. An outer face of the drying cylinder is provided with grooves, and the negative pressure is applied to these grooves through an outside suction box placed upon the cylinder. Thus, the negative pressure is transmitted through the grooves to a lower face of the cylinder, whereby a web holding force is thereby produced. This solution requires a great deal of space and the cost of construction is high.